Many audio amplifying circuits generate an audible sound when power supplies for these amplifying circuits significantly fluctuate. For example, when the amplifying circuit is initially powered, the change in the power supply may introduce a noise voltage at an output node of the amplifying circuit. This noise voltage is transferred into a sound via a load coupled to the output node, which is typically audible as a popping noise. This popping noise is a disturbance to users. It would be desirable to eliminate or at least reduce this popping noise.
FIG. 1 shows a conventional audio amplifying circuit 10. The amplifying circuit 10 is supplied by a positive power supply +Vsupply and a negative power supply −Vsupply, which are configured to provide a symmetrical supply for suppressing the popping noise. Specifically, the audio amplifying circuit 10 comprises an operational amplifier 11 with a non-inverting input node 12, an inverting input node 13 and an output node 14. The non-inverting input node 12 of the operational amplifier 11 is coupled to a first terminal 15 via an input capacitor 16. The first terminal 15 is configured to receive audio signals. The non-inverting node 12 is also coupled to ground via a first resistor 17. The inverting input node 13 of the operational amplifier 11 is coupled to ground via a second resistor 18 and to the output node 14 via a third resistor 19. The second resistor 18 and the third resistor 19 serve as a feedback network for the operational amplifier 11, and the ratio of the resistances of the resistors 18 and 19 determines an amplifying gain for the amplifying circuit 10. The output node 14 of the operational amplifier 11 is coupled to a load 20, typically a loudspeaker capable of producing sounds according to the current flowing therethrough.
While the amplifying circuit 10 is designed to suppress the popping noise, the popping noise can not be fully eliminated, especially when the amplifying circuit 10 is initially powered. FIG. 2 shows an exemplary output stage of the operational amplifier 11 of FIG. 1. As shown in FIG. 2, the operational amplifier comprises an output stage for generating an amplified output signal with high dynamic range. The output stage comprises a first PMOS transistor 21 and a second PMOS transistor 22. The first PMOS transistor 21 is coupled between the positive power supply +Vsupply and the output node 14 of the operational amplifier, and a gate of the first PMOS transistor 21 is configured to receive the input signal that is amplified by the front stage of the operational amplifier. The second PMOS transistor 22 is coupled between the gate of the first PMOS transistor 21 and the positive power supply +Vsupply, which is configured to control the operation of the first PMOS transistor 21. Due to the device structure of MOS transistors, a parasitic capacitor Cgd between the gate and drain of the first PMOS transistor 21 may be coupled into the amplifying circuit. When the amplifying circuit is initially powered, the second PMOS transistor 22 may be turned on, and then the second PMOS transistor 22 and the parasitic capacitor Cgd forms a current path permitting a differential current Idiff to flow from the positive power supply +Vsupply to the output node 14, which further flows into the ground via the load 20. The load 20 may generate a significant popping noise in response to the differential current Idiff.
Thus, there is a need for improving the noise performance of audio amplifying circuits.